Photographic methods in which a silver halide is used provide excellent sensitivity and gradational characteristic compared with other photographic methods, for example, an electronic photographic method and a diazo photographic method, and are widely used. Among the methods using a silver halide, a method for forming a direct positive image is known. The object of this method is to obtain a positive image by giving an even exposure or by using a nucleating agent in developing an internal latent image type direct positive silver halide emulsion in a surface developing solution (a developing solution for leaving an internal latent image-forming site present in the inside of a silver halide grain without substantially developing ), as shown in, for example, U.S. Pat. No. 3,761,276 and JP-B-60-55821 (the term "JP-B" as used herein means an examined Japanese patent publication). Such a direct positive silver halide emulsion has an advantage over a negative type emulsion due to its capability of providing a positive image by only one processing.
In general, an internal latent image type direct positive silver halide emulsion can be prepared via the steps of mixing a soluble silver salt and a soluble halide in an aqueous gelatin solution to form silver halide grains (core grains), followed by subjecting the core grains to a chemical sensitization and then to a silver halide deposition for forming a shell and thereafter carrying out desalting and a chemical sensitization according to necessity.
An internal latent image type emulsion which is useful as a direct positive emulsion is described in, for example, JP-B-52-34213 (U.S. Pat. No. 3,761,276). This emulsion is characterized in that it contains a doping agent in the silver halide grains and in that the surface of the grains is subjected to a chemical sensitization. This is disclosed as well in U.S. Pat. No. 3,317,322, Porter et al.
However, the internal latent image type direct positive silver halide emulsion thus prepared also has a large sensitivity reduction phenomenon (a low illuminance inversion) which is still large when compared with that of a negative type emulsion, which occurs when the exposing illuminance intensity is lowered, and there is still room for improvement.
Gelatin has been used for many years as a deflocculating agent in the preparation of silver halide emulsions. The reasons therefor are: first, it is a good protective colloid; and secondly, it is easy to handle because of easy sol--gel conversion and coating is possible. However, gelatin has defects in that it is susceptible to decomposition attributable to bacteria and fungi and in that its quality is apt to vary widely since it is manufactured from an organism. There are many unknowns regarding the unfavorable photographic effects originating in the use of gelatin and accordingly, a deflocculating agent improved over gelatin or an improved method of applying gelatin has been desired in the art.
It is already known to use a synthetic polymer in place of gelatin as a method for overcoming these problems. It is known as well to use as a deflocculating agent a polymer having a repetitive unit derived from an ethylenically unsaturated monomer containing at least one a thioether structure. Such polymers are described in U.S. Pat. Nos. 3,615,624, 3,536,677, 3,692,753, 3,690,888, 3,679,425, 3,706,564, 3,706,565, 3,713,833, 3,840,628, and 4,400,463, Research Disclosure vol. 104, pp. 44 to 48, and Journal of Imaging Science, vol. 31, pp. 148 to 156. Chemical sensitizing methods and examples of direct positive emulsions are described in these patents and publications. However, the chemical sensitizing methods described in the publications other than U.S. Pat. No. 3,690,888 are chemical sensitizations for a negative emulsion; and the direct positive emulsion described in U.S. Pat. No. 3,690,888 is a pre-fogged direct positive emulsion. Thus, there appear to be no descriptions in the literature regarding an internal latent image type direct positive silver halide emulsion which is not fogged in advance as is the case in the present invention.
In the technical field of direct positive silver halide emulsions, pre-fogged direct positive emulsions were researched and developed to provide the above mentioned internal latent image type direct positive silver halide emulsion which enables one to obtain a positive image by giving an even exposure or by using a nucleus-forming agent. In this process, the sensitivity and S/N (the ratio of signal/noise) were improved to a large extent.
In recent years, the superiority of internal latent image type direct positive silver halide emulsions has been enhanced more and more as progress has been made in the field of direct positive image-forming methods. With respect to deflocculating agents, it is expected that research on deflocculating agents will lead to improvements in the performance of internal latent image type direct positive silver halide emulsions and in the development of the applying method thereof.
Meanwhile, in internal latent image type direct positive silver halide emulsions, chemical sensitizing methods for the grain surfaces play a particularly important part and the techniques regarding chemical sensitization have been disclosed.
A method for preparing an internal latent image type core/shell emulsion which is useful as a direct positive emulsion is described in, for example, JP-A-57-136641 (the term "JP-A" as used herein means an un-examined Japanese patent application). This method is characterized in that the surface of the core/shell silver halide grains of the internal latent image type emulsion containing the silver halide grains consisting of an inner nucleus (core) of silver halide which is doped with a metal ion or chemically sensitized or subjected to both of those treatments and an outer covering (shell) of silver halide covering at least a light-sensitive site in the inner nucleus is subjected to a chemical ripening in the presence of a polymer such as poly(N-vinylpyrrolidone), poly(N-vinyloxazolidone), a vinyl alcohol, N-vinylpyrrolidone copolymer, and an N-vinyl-pyrrolidone--vinyl acetate copolymer. In JP-A-57-136641, however, only gelatin was used as a deflocculating agent in forming the grains and the improvement in low illuminance inversion characteristics was still insufficient.